Research into the specificity of the molecular features of the deltorphin class of opioid peptides utilized over 70 synthetic analogues, which included amino acid substitutions, elections, D-amino acid replacements, and derivatives, to study contributions of the "message" and "address" domains in the binding phenomenon. Our data support the contention that high affinity and extraordinary selectivity of the deltorphins for the delta receptor requires a heptapeptide with the following features: (1) a-D-amino acid in position 2 within the N-terminal sequence of Tyr-D-Xaa-Phe; (2) whereas Tyrl and Phe3 are crucial residues for binding in all deltorphins, Leu5 in deltorphin A was indispensable; (3) an appropriately positioned anionic group (Asp or Glu) is necessary for high selectivity although a negative charge per se is not required for high affinity binding, which suggests that delta affinity involves repulsion from mu sites; (4) a C-terminal amide group functions either in binding the ligand to the receptor or stabilizing Intramolecular conformation; and (5) modifications in the "message" and "address" domains differentially perturb delta and mu affinities. In particular, D-amino acid substitutions and deletion analogues, e.g., internal deletions and C-terminally derived hexa-, penta-, tetra-, and tripeptides of deltorphins A and C, with either C-terminal amide or carboxyl functions, decreased delta selectivity. In fact, tetra- and tripeptides were mu selective. These data indicate that the nature of the specific amino acid residues in the "address" domain are critical in definity of the binding properties, characteristics, and selectivity of the opioid peptides, while the "message" domain contains a general sequence for mu sites, which is universal to both delta and mu receptors.